Switched Mode Power Supplies (SMPS) are widely used in a variety of applications due to their higher efficiency compared to alternatives such as linear regulators. Some SMPSs use synchronous rectification to increase the efficiency of the rectification.
In a SMPS that does not use synchronous rectification, one or more rectifiers (such as diodes) are used on the secondary side of the SMPS to convert an Alternating Current (AC) signal propagated through the transformer into a Direct Current signal at a regulated voltage or current level. Voltage drops across the one or more diodes, however (such as a 0.7 volt (V) drop across a forward-biased silicon diode) are a source of loss.
To reduce the loss due to rectification, synchronous rectification uses a switching transistor (such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or an enhancement mode gallium arsenide transistor) instead of a diode. The switching transistor, although it has an inherent body diode that would rectify the AC signal, is controlled to operate more efficiently by switching the switching transistor on when current flows in an appropriate direction, and switching the switching transistor off otherwise. Because a typical switching transistor has an on-resistance on the order of milliohms or a few tenths of a milliohm, for most power converter applications the voltage drop across the switching transistor when it is switched on is much lower than, for example, the voltage drop across a Schottky diode. Lowering the voltage drop lowers the losses caused by the voltage drop, and therefore improves the efficiency of the SMPS.
Control of the switching transistor rectifier is typically based on current through the switching transistor. During a normal conduction phase, when the stitching transistor is not yet turned on a voltage drop builds across the switching transistor, and some leakage current may pass through the body diode of the switching transistor. When the voltage drop is sensed as being greater than a first predetermined threshold, the switching transistor may be switched on. When the voltage across the switching transistor is sensed as dropping back to below a second predetermined threshold (for example, OV), this may indicate the end of the conduction phase, and the switching transistor is turned back off to provide proper rectifier operation.
However, noise resulting from switching transients can result in false triggering of the switching transistor, leading to inefficient operation. False triggering can be alleviated by the use of minimum on and off times, which allow transients to dissipate before the sensing of the voltage drop. Thus, transients that occur during these “blanking” intervals are ignored and have no effect on the switch control of the switching transistor rectifier.
Furthermore, reactive components in the SMPS, such as inductors and capacitors, can cause distortions of the sensed voltage drop across the switching transistor.
The noise and distortions may cause a premature sensing of the end of the conduction phase, and a corresponding premature turn off of the switching transistor, that is, the switching transistor may be turned off well before the conduction phase ends.
The body diode of the switching transistor continues to conduct during the conduction phase when the switching transistor is turned off. Because the body diode has a higher voltage drop than the voltage drop across the turned-on switching transistor, prematurely turning off the switching transistor causes an increase in losses due to the voltage drop of the rectification circuit and reduces the efficiency of the SMPS.
Accordingly, there is a need for a method and an apparatus for self-synchronous rectifier control that prevent efficiency losses due to conditions that may cause a premature turn-off of the switching transistor.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the embodiments.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments. This serves to not obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the disclosures herein. The details of well-known elements, structures, or processes that are necessary to practice the embodiments and that are well known to those of skill in the art may not be shown and should be assumed to be present unless otherwise indicated.